Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.1) Gecko/20090624 Firefox/3.5 (.NET CLR 3.5.30729)
I got to 342 MB before I decided not to continue. Can you use an addon like Live HTTP Headers to see what headers Firefox is sending to the server when you attempt to download?

To comment #2, I used 100GB NTFS partition.
The same partition was used for the sample "wget" run in my post.
To comment #1, I will install an addon to see the header.
BTW, so your FF3.5 downloaded more than 342MB and still kept on
continueing? Hmm, could it be related to I18N/L10N issues in a mysterious way?
Anyway, I will my header findings later.

I installed "Live Header" addon.
It is quite nifty.
And I found out the culprit very quickly.
Below is the header when I visited the 4.3GB and found that
it got truncated.
Note the " Via: 1.1 winxp.example.org:3128 (squid/2.7.STABLE6)"
header line in the final OK response.
It seems to be that the proxy chain (I use privoxy and squid due to
historical reasons) and somewhere the connection is cut
at 0.3GB.
When I turned off proxy and accessed the mentioned URL directly from FF3.5,
it went past the previous limit and kept on copying.
So, this bug can be closed. I have to check the proxies and
then report the problems up stream.
Sorry for my premature investigation, and thank you for the suggestion to use Live Header addon.
(These proxies worked like a charm. The last time they caused problems was
several years ago when junkbuster didn't handle HTTP/1.1 connection. You can
tell it was a long time ago in the Internet time scale.)
http://ring.nict.go.jp/archives/linux/knoppix/iso/knoppix_v5.3.1DVD_20080326-20080523-AC.iso
GET /archives/linux/knoppix/iso/knoppix_v5.3.1DVD_20080326-20080523-AC.iso HTTP/1.1
Host: ring.nict.go.jp
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; ja; rv:1.9.1) Gecko/20090624 Firefox/3.5 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-2022-JP,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Proxy-Connection: keep-alive
Referer: http://ring.nict.go.jp/archives/linux/knoppix/iso/
HTTP/1.x 200 OK
Date: Sat, 04 Jul 2009 00:13:25 GMT
Server: Apache/2.2.11 (Unix)
Last-Modified: Sat, 24 May 2008 23:37:47 GMT
Etag: "79c9e1-114765000-44e026f0f08c0"
Accept-Ranges: bytes
Content-Length: 4638265344
Content-Type: application/octet-stream
X-Cache: MISS from winxp.example.org
X-Cache-Lookup: MISS from winxp.example.org:3128
Via: 1.1 winxp.example.org:3128 (squid/2.7.STABLE6) <=== AHA!
Connection: close
Proxy-Connection: keep-alive